Prior to the introduction of the IIe with it's capability to display what they called "double hi-res" graphics, the II series had only an 8 color palette in hi-res, and a 16 color palette in lo-res (which was too blocky for most games). Out of those 8 colors, there were 2 different blacks, and 2 different whites, along with green, violet, orange and blue. Any games that needed more colors in hi-res used dithering to create the effect. With double hi-res, you had more colors available (I think), but it took a while for game developers to create many titles that used double hi-res, because they couldn't be run on any of the older II series machines. In a nutshell, that's why.